Animate a plane that transforms to a mesh?
I just found out this technique from a tutorial that uses another software.
But it applies on Blender:


create a plane and subdivise it so it has enough vertexes (eg. 10 times)
add an empty at the same place, and set the rotation to X:90°
add a SimpleDeform modifier on the plane


Switch mode to Bend
Set Axis, Origin to your empty
Set Deform Angle to 360° so it becomes a tube

duplicate your empty and sets its rotation to X:90° / Z:90°
add a second SimpleDeform modifier on the plane


Switch mode to Bend
Set Axis, Origin to your second empty
Set Deform Angle to 360° so it becomes a sphere !



Et voilà ! You can now animate both deform angle from the SimpleDeform Modifiers to animate it.